Pulp screening in the pulp and paper industry is generally achieved with pressure screens in which the furnish is pumped to remove oversized contaminants, fiber bundles, wood fragments, and/or sort long and short fibers, and/or stiff and soft fibers. Several pressure screen configurations are in use today. The two main configurations are pressure screens using substantially cylindrical outward flow basket, and screens using substantially cylindrical inward flow basket. All the pressure screens generally require the accepted stock to pass through the holes or slots of a screen basket. Rejects too large to pass through the screen basket or wedge-wire basket openings leave through a rejects outlet. A rotor is used to prevent the slots or holes from plugging. The rotor creates pressure pulsations resulting in sufficient cleaning action of the screening surface to avoid plugging condition.
In applications with aggressive rotor, screen baskets are subjected to high dynamic loading from the pressure pulsations generated by the rotor. In such applications, ring fatigue failure of wedge wire basket is commonly observed and the mills are often forced to get back to milled slot or drilled baskets, resulting in a loss of capacity and/or efficiency for obtaining acceptable screen basket life.
Wedge wire baskets are most commonly welded assemblies. Baskets from these constructions suffer to various degrees from distortion, residual stress, stress concentration from welded joint geometry and weld defects, which can normally be associated with most welding processes.
Among the screen baskets commercially available, the wedge wire baskets are the ones offering the highest open area because of their continuous slots extending over the entire basket length. The accuracy of the cold drawing process used in wire manufacturing can be advantageously used to achieve exceptionally accurate slot width and highly repeatable contour geometry, which contribute in providing better efficiency and capacity than milled slot or drilled baskets.
Conventional wedge wire screen baskets generally comprise a plurality of wedge wires (profiled bars) that are permanently joined to support rings by various methods. It should be understood by the term “permanently joined” that a destructive method, such as cutting, grinding, chemical attack, etc., would be required to remove the profiled bars. Support rings are necessary to withstand the loading to which the basket is subjected during screening. For severe applications, the screen basket are often reinforced using additional welded or shrink fit rings or backing jacket.
Some efforts have been made by manufacturers to re-use a portion of the main structure by using replaceable screening elements. However, the manufacturing of these elements is costly and requires several steps and more material at each rebuild. Also, loss in open area is sometimes generated because of the fixing devices such as cage or backing jacket. Finally, the slot and contour accuracy and uniformity are also compromised because of the assembly method. An example of a wedge wire screen basket which has its main structure re-use can be seen in International Patent Application No. WO 02/083263 A1, published Oct. 24, 2002, naming Lutz et al. as the inventors and titled “Screen Cylinder and Method”.
U.S. Pat. No. 5,791,495, issued to Gero et al. on Aug. 11, 1998 and titled “Paper Pulp Screen Cylinder” describes a basket made with milled bars to obtain a basket with replaceable profiled bars. However, the proposed basket is not provided with continuous slots over the entire basket length, which compromises its capacity. Also, the manufacturing of the profiled bars adapted for the proposed screen basket requires additional steps, such as milling, cutting, and machining. Furthermore, the quantity of parts to be assembled becomes very high because each row requires a set of milled bars.
U.S. Pat. No. 6,051,103 issued to Aikawa on Apr. 18, 2000 and titled “Paper-Making Screen Apparatus” concerns an inward-flow screen basket provided with both replaceable bars and continuous slots. A drawback of Aikawa's basket is that a minimum clearance has to be provided between the rod-shaped members and the engaging sections of the support members to allow insertion of the rod-shaped members therein. This clearance may cause the movements of the rod-shaped members in the engaging sections during operations and even vibrations, which in turn can cause fatigue and premature wear of the parts.